how does that competition work? each person gets the track to themselves for the best time or does everyone who wants to participate go on at once?

I was there for one of the BMW competitions previously. You just run the group you sign up with and then at the end of the day they look at which BMW's have the fastest lap times and those are the ones that win. If enough BMW's sign up then Persian54 may be able to convince them to have a BMW only run group but it didn't happen last time.